Fix VS Code MCP server initialization crash and multi-agent architecture

CRITICAL FIX:
- Removed auto-registration during initialization that was causing crashes
- Fixed pattern match error with Inbox.start_link (already_started case)
- Changed architecture to require explicit agent registration with unique IDs

MULTI-AGENT SUPPORT:
- Agents must now register themselves with unique identifiers (e.g., 'Green Platypus')
- register_agent tool now works without requiring prior agent_id
- All other tools require agent_id parameter to identify the calling agent
- Proper error handling for missing agent_id in tool calls

ARCHITECTURE CHANGE:
- One MCP server instance serves multiple agents (as per VS Code design)
- Removed auto-registration of 'GitHub Copilot' agent during initialization
- Each agent must explicitly call register_agent before using other tools

This fixes the VS Code connection error:
MatchError: no match of right hand side value: {:error, {:already_started, #PID<0.215.0>}}

mcp_servers.json

@@ -10,12 +10,6 @@
             "auto_restart": true,
             "description": "Context7 library documentation server"
         },
-        "mcp_figma": {
-            "url": "http://127.0.0.1:3845/mcp",
-			"type": "http",
-            "auto_restart": true,
-            "description": "Figma design integration server"
-        },
         "mcp_filesystem": {
             "type": "stdio",
             "command": "bunx",